When a chemical reaction occurs, bonds are broken and new bonds are formed and products have a different identity from the reactants. In this lesson, we will discuss one of the main types of chemical reactions, called a double displacement reaction.
What Is a Double Displacement Reaction?
Let's imagine ourselves going to a dance and having a dance partner. Once we arrive, we interact with the other people, and we end up switching dance partners with another person. Now you end up with a new dance partner. The same kind of thing can be compared to what occurs in a double displacement reaction in chemistry.
A double displacement reaction, also known as a double replacement reaction or metathesis, is a type of chemical reaction where two compounds react, and the positive ions (cation) and the negative ions (anion) of the two reactants switch places, forming two new compounds or products. Here, you can see the general form of a double displacement reaction:
How to Complete a Double Displacement Reaction
Just like how dance partners can be switched, the products of a double displacement reaction are the result of the cations and anions of the reactants trading partners with each other. Now we'll learn the steps to complete and predict the correct products for a double displacement reaction. Let's start with a look at the chemical reaction between Na 2 S and HCl.
Step 1: Identify the Individual Ions from the Reactants and Their Charges
For the reactant Na 2 S , there is a cation (positive ion) and an anion (negative ion). Na is written first, so that means Na is the cation. S is written second, so S is the anion. Cations and anions have charges that are either positive or negative integers. A cation has a positive charge, and an anion has a negative charge.
S has no subscript, which is just the small numbers at the bottom right after each element. No subscripts mean that the subscripts are equal to one. This means that if we rewrite the compound Na 2, the subscript for S is 1. We can then reverse the subscripts and figure out the individual charges of Na and S. Na has a charge of +1, and S has a charge of -2, as shown here:
Now, let us take a look at the other reactant, HCl. The cation is H, and the anion is Cl. Like we did with Na 2 S , we reverse the subscripts to figure out the individual charges.
Step 2: Switch the Cations and Anions of the reactants
To predict the products, bring down the charges as shown in this illustration:
Originally, the pairs are Na-S and H-Cl. Now, the new partner of Na is Cl, and the new partner of H is S.
Step 3: Balance the Chemical Reaction
The reaction in this case is not balanced, because the number of Na and H atoms (in red) is not the same.
What we need to do next is to balance the reaction by adding coefficients, which are the numbers before each compound. In this case, we will put a 2 in front of HCl to balance the H atoms and a 2 in front of NaCl to balance the Cl atoms.
Now, we have successfully balanced the reaction.
Examples of Double Displacement Reactions
For our first example, we'll look at the reaction between Li 2 SO 4 and BaCl2 . SO4 is a polyatomic ion, so we will treat this as one anion. The ion SO4 does not have a subscript (4 is not the subscript because this is included in the ion). The subscript of SO4 is 1. Let's first determine the ions and their charges.
Now, we can switch the ions and come up with the double displacement reaction.
For our next example, let's look at the reaction between NaOH and CaBr2 . For this reaction, OH is a polyatomic ion and is treated as one whole anion. The individual ions and the charges for these two reactants are demonstrated in this image:
Now, we can switch the ions, as you can see here:
Types of Double Displacement Reactions
Now that we have gone over the steps to complete and balance double displacement reactions, let us go over the different types of double displacement reactions. There are three types of double displacement reactions: precipitation, neutralization and gas formation. We will discuss each and go over an example for each.
A precipitation reaction is when two compounds react and form a precipitate, which is a solid product. This product is insoluble, or cannot be dissolved in water.
Over 79,000 lessons in all major subjects
Get access risk-free for 30 days,
just create an account.
For example, we have the reaction between NaCl and AgNO3 . When the two solutions, NaCl and AgNO3 , are mixed, a precipitate, AgCl, is formed. The subscript aq means aqueous, which means it is a solution in a solvent. The subscript s means solid. These subscripts indicate the phase of the compounds.
Going over the steps we outlined earlier to complete this double displacement reaction, we obtain the final reaction at the bottom of this image. The precipitate formed is AgCl, as indicated by the subscript s. The resulting reaction is already balanced, so there is no need to add coefficients.
Next we have a neutralization reaction, which is when an acid reacts with a base. The result is water and a salt. A salt is an ionic compound, which is the result of a combination between a cation and an anion.
An example of a neutralization reaction is the reaction between NaOH (base) and HCl (acid).
The salt formed is NaCl. Water, H 2 O, has a subscript of l, which indicates that it is in the liquid phase. The resulting reaction is already balanced, so there is no need to add coefficients.
Gas Formation Reaction
Finally, we have a gas formation reaction, which occurs when one of the products is in a gaseous state. This will be indicated by a product that has a subscript of g, which stands for gas. The initial product will actually be in the aqueous state, but this will break down because it is unstable. This breakdown results in a gas. The usual aqueous compounds that break down to form gaseous compounds are shown on screen:
For our example, let's look at a reaction between HNO3 and Na2 SO 3.
The products are initially NaNO3 and H2 SO3. However, as we mentioned above, H2 SO3 is unstable and further decomposes to H2and SO2 . The gas formed is SO2 .
Let's review. A double displacement reaction, also known as a double replacement reaction or metathesis reaction, is a type of reaction that occurs when the cations and anions switch between two reactants to form new products. When predicting products in a double displacement reaction, we need to identify the different ions first and their charges. After that, we switch the ions of the two reactants and write down the products. The final step is to make sure that your chemical equation is balanced.
There are three types of reactions that fall under the double displacement reaction category: precipitation, neutralization and gas formation. A precipitation reaction forms an insoluble solid compound. A neutralization reaction occurs between an acid and a base that forms a salt and water. A gas formation reaction is when two compounds react to form a gaseous product.
After reviewing this lesson, you'll have the ability to:
Define double displacement reaction
Explain how to predict the products in a double displacement reaction
Describe the three types of reactions that are considered double displacement reactions
Double displacement reactions occur when two aqueous solutions react to form either water, a solid (insoluble compound) or a gas and an aqueous solution. Let's work on predicting and balancing the reactions.
Problems for More Practice
Write the balanced chemical reaction for each scenario, which includes predicting the products of the reaction.
1) Sodium oxide reacts with silver acetate.
2) Silver nitrate reacts with magnesium chloride.
3) Potassium carbonate reacts with ammonium iodide.
4) Cesium sulfide reacts with sodium hydroxide.
5) Sodium hydroxide reacts with hydrochloric acid.
1) Na2 O + 2AgC2 H3 O2 ⟶ 2NaC2 H3 O2 + Ag2 O
Did you know… We have over 200 college
courses that prepare you to earn
credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the
first two years of college and save thousands off your degree. Anyone can earn
credit-by-exam regardless of age or education level.